A body implantable, transvenous lead may be used to electrically connect a pulse generator, such as a pacemaker, with body tissue, such as that of the heart, to be stimulated. A lead of this kind typically includes a lead body comprising a tubular, flexible insulative sheath or housing of silicone rubber, polyurethane or other suitable biocompatible, biostable polymer. In a conventional bipolar cardiac pacemaker lead having a tip electrode and a ring sensing electrode, a pair of metallic coil conductors arranged coaxially with insulation in between the conductors are carried within the insulative housing. One of the coil conductors connects the pulse generator with the tip electrode while the other coil conductor, somewhat shorter than the first conductor coil, connects the pulse generator with the ring sensing electrode positioned proximally of the tip electrode.
Transvenous pacemaker leads often combine a cardioverting and/or defibrillating capability with the pacing and sensing functions. Thus, besides pacing and sensing electrodes, a transvenous type lead may include along its distal end portion one or more cardioverting and/or defibrillating electrodes for shocking selected tissue, for example, the tissue of the superior vena cava (SVC) or the tissue of the right or left ventricle.
To reduce the outside diameter of transvenous leads, lead bodies comprising multilumen housings have been developed. In place of coil conductors, such multilumen housings may contain multistrand metal cable conductors to connect the pulse generator at the proximal end of the lead with the various stimulating and/or sensing electrodes along the distal end portion of the lead body. In some existing multilumen housing lead bodies, a combination of one or more coil conductors and one or more cable conductors is utilized.
A significant portion of the cost of leads utilizing metallic coil or cable conductors is associated with such metallic conductors that require labor-intensive fabrication processes and expensive termination techniques such as welding or crimping. Further, metallic conductors are subject to fatigue failure, even in coil form. In addition, changing electrode placement and/or spacing or changing the length of the lead requires the modification of several parts of the lead, making lead customization complex and costly.
Conductive polymers have been used in body implantable, tissue stimulation leads. For example, U.S. Pat. No. 5,681,514 discloses a body implantable lead comprising alternating layers of conductive and insulative thermosetting polymers with the conductive polymer layers serving as the electrical conductors of the lead. The lead is fabricated by extruding the alternating conductive/insulative polymer layers through successive, coaxially arranged heated nozzles of increasing outer diameter. The '514 patent describes conductive polymers as including polymers filled with electrically conductive particles, intrinsically conductive polymers, and doped polymers.
Body implantable stimulation leads carrying electrodes made of conductive polymers are also known. For example, the above-mentioned '514 patent describes the formation of conductive polymer tip and ring electrodes during the extrusion process by controlling the extrusion of the various layers independently to selectively expose portions of the conductive polymer, with the exposed portions of the conductive polymer serving as the electrodes. Further, U.S. Pat. No. 5,476,496 discloses a bipolar body implantable pacing lead including inner and outer, coaxial metal coil conductors. The inner coil conductor is electrically connected to a pacing/sensing tip electrode at the distal extremity of the lead. The coils are electrically isolated from each other by a first insulating sleeve disposed between the inner and outer coil conductors. The outer coil conductor is enclosed within a second insulating sleeve. A portion of the second sleeve is made of a conductive polymer engaging the outer coil conductor so as to provide electrical communication between the sleeve and the coil conductor. The conductive polymer portion of the second sleeve functions as the indifferent electrode of the pacing system. Still further, U.S. Pat. No. 3,815,611 discloses an isodiametric body implantable lead incorporating a proximal cardioverting electrode constructed of a conductive, flexible silicone rubber material.
The advantages of providing pacing therapies to the left side heart chambers and to both the right and left heart chambers are well established. For example, in four chamber pacing systems, four pacing leads, typically bipolar leads, are positioned for both pacing and sensing in or on the respective heart chambers. To provide left side stimulation and sensing, leads are transvenously implanted in the coronary sinus region, for example, in a vein such as the great vein or the left posterior ventricular (LPV) vein proximate the left ventricle of the heart. Such placement avoids the risks associated with implanting a lead directly within the left ventricle which can increase the potential for the formation of blood clots which may become dislodged and then carried to the brain where even a small embolism could cause a stroke. (As used herein, the phrase “coronary sinus region” refers to the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other vein accessible by way of the coronary sinus.)
The tip electrode of a lead implanted in the coronary sinus region can pace and sense left side ventricular activity. When such a lead includes a ring electrode proximal of the tip electrode and residing in the coronary sinus above the left ventricle closely adjacent to the left atrium of the heart, pacing and sensing of left atrial activity is made possible. Moreover, the lead may include one or more electrodes for the delivery of electrical shocks for terminating tachycardia and/or fibrillation. Such cardioverting and/or defibrillating electrodes may be used by themselves or can be combined with the aforementioned pacing and/or sensing electrodes.
The implantation of a lead through the coronary ostium and into the veins in the coronary sinus region is often difficult because of the extreme curvatures in the coronary vessels, their narrowness, anomalies in the vascular anatomy because of disease, and the number of veins which may communicate with the desired lead feed path. Some currently available leads, and particularly the distal end portions thereof, are too stiff and/or too large in diameter to permit easy maneuvering of the distal end portion within the coronary vessels.
Thus, there is a need for improved body implantable leads that simplify and reduce the costs associated with the conductive path or paths thereof, that facilitate the customization of electrode placement and spacing, and provide the flexibility and small diameters needed to enable the lead to be tracked through the coronary ostium and into the veins of the coronary sinus region for left side pacing, sensing and/or cardioversion/defibrillation.